Problems: The biggest threats to carrots are four-footed critters such as deer, gophers, woodchucks, and rabbits. For controls, see the Animal Pests entry. Otherwise, carrots are fairly problem free.
Keep an eye out—particularly in the Northwest—for carrot rust flies, which look like small green houseflies with yellow heads and red eyes. Their eggs hatch into whitish larvae that burrow into roots. Infested roots turn dark red and the leaves black. Infestations usually occur in the early spring, so one solution is to delay planting until early summer, when damage is less likely. Or cover plants with a floating row cover to keep flies away.
Parsleyworms are green caterpillars with black stripes, white or yellow dots, and little orange horns. They feed on carrot foliage, but they are the larval stage of black swallowtail butterflies, so if you spot them on your carrots, try not to kill them. Instead, transfer them to carrot-family weeds such as Queen Anne’s lace, and watch for chrysalises to form, and later, beautiful butterflies!
The larvae of carrot weevils, found from the East Coast to Colorado, tunnel into carrot roots, especially in spring crops. Discourage grubs by rotating crops.
Nematodes, microscopic wormlike animals, make little knots along roots that result in stunted carrots. Rotate crops and apply plenty of compost, which is rich in predatory microorganisms. (For more controls, see Plant Diseases and Disorders.)
Leaf blight is the most widespread carrot disease. It starts on leaf margins, with white or yellow spots that turn brown and watery. If leaf blight is a problem in your area, plant resistant cultivars.
Hot, humid weather causes a bacterial disease called vegetable soft rot. Prevent it by rotating crops and keeping soil loose. The disease spreads in storage, so don’t store bruised carrots.
Carrot yellows disease causes pale leaves and formation of tufts of hairy roots on the developing carrots. The disease is spread by leafhoppers, so the best way to prevent the problem is by covering new plantings with row covers to block leafhoppers.
Harvesting: Carrots become tastier as they grow. You can start harvesting as soon as the carrots are big enough to eat, or leave them all to mature for a single harvest. Dig your winter storage crop before the first frost on a day when the soil is moist but the air is dry. Since spading forks tend to bruise roots, hand-pull them; loosen the soil with a trowel before you pull. Watering the bed before harvesting softens the soil and makes pulling easier.
Carrots are excellent to eat both fresh and cooked. Note that purple-rooted varieties will lose their purple pigment if cooked in water, but tend to keep it when roasted.
To save harvested carrots for winter use, prepare them by twisting off the tops and removing excess soil, but don’t wash them. Layer undamaged roots (so they’re not touching) with damp sand or peat in boxes topped with straw. Or store your fall carrot crop right in the garden by mulching the bed with several inches of dry leaves or straw.